The present invention relates to antenna tuning systems, being more particularly, though not exclusively, concerned with automatic and synchronous tuning of very low frequency (VLF) antennas to obviate the sensitivity of solid state or similar transmitters generating frequency shift-keyed modulation signals or the like to the deleterious effects of load mismatch caused by detuning, such that mismatched loads may be effectively and dynamically fed with high efficiencies.
The VLF band (say, 3-30 kHz) offers certain unique characteristics for reliable, worldwide communications. Propagation occurs mainly in the vertically polarized, ground-wave mode, which experiences very little attenuation as it travels in the earth-ionosphere waveguide, and is little affected by the ionospheric disturbances which often interrupt higher-frequency traffic. Unfortunately, however, VLF is difficult to radiate from ground-based antennas, the dimensions of which are limited, by practical necessity, to a small fraction of a wavelength. Excitation of an electrically short antenna requires the transmitter to supply large amounts of reactive power to localized, non-radiating modes. The reactive power may be hundreds of times the real power supplied, and owing to resistive losses which occur in the soil close to the antenna, only a fraction of the real power gets radiated.
VLF transmitting antennas typically consist of an insulated network of wires supported by towers at the highest practical altitude, fed from the ground by an insulated tower or downlead. The effective antenna height is essentially the physical height. The local electrostatic mode appears as a capacitance in series with the radiation and ground resistances; but the capacitive reactance can be cancelled at a single operating frequency with an appropriate series inductance. The Q of the resulting series-tuned circuit is often several hundred, and the resulting -3 dB bandwith may be inconveniently narrow for frequency-shift-keyed (FSK) data transmission, especially at the lower frequencies. Until the advent of the present invention with its dynamic tuning, communication system designers were forced to accept VLF/LF antenna-system bandwith as a fundamental limitation on FSK signaling rates. Ground-based transmissions have been restricted to a few hundred baud, while airborne stations with their broader antenna bandwidths have operated at over a thousand baud.
While the simplest way to expand antenna system bandwidth is by adding series resistance, this approach degrades efficiency, and is economical only when a small improvement in bandwidth is needed. The only other practical means for bandwidth enhancement involves electronically varying the tuning inductance so that the antenna resonance tracks the excitation frequency provided by the transmitter. A variety of techniques for accomplishing this function have been tried, including permeability tuning, using a very large ferrite core (Jacob, M. I. and Brauch, H. N. "Keying VLF transmitters at high speed", Electronics, Vol. 27 (1954) No. 12, pp 148-151); and SCR switching with square-loop magnetic cores by the assignee of the present invention, "Controlled Antenna Reactor for Bandwidth Enhancement (CARBE)", RADC TR-74-161 (July 1974). In such prior dynamic tuning, one of the ferrite reactors, as described in the above Electronics article, is used in each of the two helix houses to increase the antenna bandwidth to 72 Hz, so that the radiated power on 17.8 kHz with 100 Hz frequency shift is about twice that obtained without dynamic tuning. Each reactor is housed in a large oil tank weighing 23 tons of which 3.75 tons are ferrite material. Though drive-circuit limitations have prevented the reactors from reaching their design levels of performance, at least the practical utility of dynamic tuning has been demonstrated.
More efficient drivers based on the use of SCRs and resonant charging have been developed, and it has been discovered that square-loop magnetic material may be operated under large-signal condition as a two-state switch. Far less core weight and volume is needed if the magnetic core is operated as a two-state switch. As an example, radiation of 1600 baud MSK with a 100 kW transmitter on 37.2 kHz has been achieved. The antenna bandwidth was less than one-fourth the frequency shift--truly an otherwise impossible signaling situation without the dynamic tuning of the present invention. Moreover, a modular dynamic tuning system has been achieved for VLF stations with dynamic tuning suited to the station's individual requirements, so that 200 baud signals may be transmitted efficiently on each assigned frequency, and with the frequency shift (100 Hz) and system bandwidth (160 Hz) comparable, so that reduced stress on vacuum-tube transmitters and greatly improved dc-to-rf conversion efficiency is achieved.
These benefits are particularly significant also in the development of solid-state VLF transmitters which are especially sensitive to load mismatch caused by detuning; with much of their promised operating efficiency and reliability otherwise being lost unless the antenna is synchronously tuned.